One type of active control system in which the invention is particularly useful is an active acoustic attenuation system. Acoustic attenuation includes sound attenuation and vibration attenuation. In active acoustic attenuation, a canceling acoustic wave destructively interferes with and cancels an input acoustic wave to yield an output acoustic wave. An adaptive control filter in an active acoustic attenuation system inputs a reference signal, and in turn outputs a correction signal to an output transducer. The output transducer is normally a loudspeaker in a sound attenuation system or shaker in a vibration attenuation system. The output transducer receives the correction signal and outputs a canceling acoustic wave to destructively interfere with the input acoustic wave so that an output acoustic wave is zero or some other desired value.
The output acoustic wave is sensed with an error sensor such as a microphone in a sound system or an accelerometer in a vibration system. The error sensor transmits an error signal that is used to update adaptive weights in the control filter.
Under some circumstances, it may be beneficial to selectively constrain adaptation of the adaptive control filter, or even constrain the output from the output adaptive control filter. For example, this may be desirable when the output of physical components in the active control system is not reliable beyond certain frequency ranges.